Refactor MobileNetBackbone for readability

[jbischof]

Our backbone modeling files are very large and hard to read. @LukeWood proposed two changes in #1745:

1. move aliases to their own file
2. move helpers below model declarations

#1745 provides a nice template for this change but sat around for a long time and now has big merge conflicts. Replicate this change rebased on the current master for keras_cv/models/backbones/mobilenet_v3/.

[ID6109]

I'll be starting/resuming work on #1637 , #1635 , #1634 , and #1628 next week. I'll implement the improvement suggested in #1745 in the PRs for these anyway, so how about I take up this, #1807 , #1808 , and #1809 as well? If all goes to plan, we should have ready-to-review PRs for all the issues mentioned by the end of the month.

[jbischof]

I assigned you #1806 but can you also comment on #1807, #1808, and #1809? Otherwise it won't let me. Thanks!

[jbischof]

@ID6109 really appreciate your help! Re: backbone conversions let's take one or two at time since it would take too long for a single person to do all of them.

[ID6109]

@jbischof I have #1642 , #1643 , #1649 , #1759 , and #1829 open for these conversions. Should I pause my work for #1635 , #1634 , and #1628 ?

[jbischof]

@ID6109 my recommendation would be to take only the number/scope of issues that you can actively work on and make sure the issue is assigned to you. This allows us to keep tabs on what's being worked and close/reassign anything stale. Again, appreciate your work!

[ID6109]

Thanks @jbischof! I'm comfortable with simultaneously working on my open PRs as of now. Most of these issues have been assigned to me, and I am taking up the rest because they have gone stale. I've earlier commented for #1649 (on #1630) and #1829 (on #1637). I'll also comment on #1635 and #1628 so they're formally assigned to me.

[jbischof]

@ID6109 I think the right strategy is to get assignments for what you can work on in the next week or so and then pick up more if you have time. This is a TON of work 😅

[ID6109]

I have PRs open for all the issues assigned to me. I'll focus on the comments I get on these for now rather than going for more. Thanks!